Ring-halogenated .alpha.-methylstyrenes are intermediates for the preparation of plant-protection agents (see, for example, U.S. Pat. No. 4,211,548). U.S. Pat. No. 4,594,467 describes a process in which 3,5-dichloro-.alpha.-methylstyrene is prepared by the dehydrobromination of .alpha.-bromo-3,5-dichlorocumene with an alkali metal hydroxide solution in the presence of a phase-transfer catalyst above 70degrees Centigrade (.degree.C.). While this process enjoys excellent yields, it has the disadvantage of requiring the use of expensive and often toxic phase-transfer catalysts that must be removed after completion of the reaction. The separation of catalyst from the reaction mixture often requires multiple unit operations. After separation, the phase-transfer catalyst requires recycle or disposal. It would be clearly beneficial to have an efficient process that eliminates the need of the phase-transfer catalyst.
Furthermore, the process described hereinabove utilizes a caustic or basic medium such as, for example, sodium hydroxide. Such caustic solutions are incompatible with glass-lined vessels. Again, it is highly desirable to have a process that does not require the addition of a base to accomplish the desired elimination.